cr ratios and ign timing

[blacknovagsi]

hiya not posted for a while, but i was reading some of the posts about cr ratios and wondered how do you work it out? i've got a 1.6 8v and the pistons are flat topped as opposed to having recesses in them like the gte should have, my bottom end is from a 2004 astra and the head is nova gsi. also which way do i move the distributor to advance/retard the timing and what would be a good base setting till i get it set up properly:tard:. cheers people, long live the nova!!!!!!!

[MC]

Unless you measure, cr's are just a guesstimation. As most Nova 8v engines are similar being that the majority of the squish volume is in the head I use the following as a rule of thumb:

Example 1600 bottom end and 1300 cylinder head.

1600 compresses to approximately 160cc (10:1)

1300 compresses to approximately 137cc (9.5:1)

The compressed value would be the squish chamber. Add the two together and you get roughly:

1600 down to 137cc and a ratio of 11.7:1

Like I say its a guesstimation, I take other stuff into account and normally round it off. Having different piston like yours throws a spanner in!

MC

[Stuart]

read the 1.4 tuning article in the articles section.... tells you howto measure your CR

[mowgli]

rotating the dizzy towards the windscreen will advance it...

[ck]

as above. looking from the passenger side at the dizzy, turn clockwise to advance..